Hello Rafaela. Erik is right about that you can evolve your system from AC450 to AC800M if you subscribe on Sentinel.
If you have, or buy a Sentinel subscription for your system, you can evolve the AC450 licence to AC800M free of charge.
According to our mapping guide you get one PM891 CPU license if you have one AC450 license. You get a license for the same level of redundancy and number of communication interfaces etc, as you currently use in your AC450.